There exist two commonly implemented front-end architectures in radio frequency (RF) receiver design; namely, the homodyne architecture and the heterodyne architecture. The homodyne architecture down-converts a desired channel directly from RF to baseband, whereas the heterodyne architecture down-converts a desired channel to one or more intermediate frequencies (IF) before down-conversion to baseband. In general, each of these front-end architectures employ an antenna to receive an RF signal, a band-pass filter to suppress out-of-band interferers in the received RF signal, a low noise amplifier (LNA) to provide gain to the filtered RF signal, and one or more down-conversion stages.
Each component in a receiver front-end contributes noise to the overall system. The noise of a component can be characterized by its noise factor (F), which is given by the ratio of the SNR at the input of the component to the SNR at the output of the component:FCOMPONENT=SNRIN/SNROUT The noise of the overall receiver front-end increases from input to output as noise from successive components compound. In general, the overall noise factor of the receiver front-end is proportional to the sum of each component's noise factor divided by the cascaded gain of preceding components and is given by:
      F    TOTAL    =            F      1        +                            F                      2            -            1                          -        1                    A        1              +                            F                      3            -            1                          -        1                              A          1                ⁢                  A          2                      +    …    +                            F                      n            -            1                          -        1                              A          1                ⁢                  A          2                ⁢                                  ⁢        …        ⁢                                  ⁢                  A                      n            -            1                              where Fn and An represent the noise factor and gain of the nth component in the receiver front-end, respectively. The above equation reveals that the noise factor (F1) and gain (A1) of the first gain component can have a dominant effect on the overall noise factor of the receiver front-end, since the noise contributed by each successive component is diminished by the cascaded gain of the components that precede it.
To provide adequate sensitivity, therefore, it is important to keep the noise factor (F1) low and the gain (A1) high of the first gain component in the receiver front-end. The sensitivity of the receiver front-end determines the minimum signal level that can be detected and is limited by the overall noise factor of the receiver front-end. Thus, in typical receiver designs the first gain component in the front-end is an LNA, which can provide high gain, while contributing low noise to the overall RF receiver.
LNAs provide relatively linear gain for small signal inputs. However, for sufficiently large input signals, LNAs can exhibit non-linear behavior in the form of gain compression; that is, for sufficiently large input signals, the gain of the LNA approaches zero. LNA gain compression is a common issue confronted in RF receiver design, since large out-of-band interferers referred to as blockers can accompany a comparatively weak desired signal in a received RF signal. For example, in the Global System for Mobile Communications (GSM) standard, a desired signal 3 dB above sensitivity (−102 dBm) can be accompanied by a 0 dBm blocker as close as 80 MHz away. If these large out-of-band interferers are not attenuated prior to reaching the LNA, they can reduce the average gain of the LNA. As noted above, a reduction in the gain provided by the LNA leads to an increase in the noise factor of the receiver front-end and a corresponding degradation in sensitivity.
Therefore, a band-pass filter is conventionally employed in the receiver front-end, before the LNA, to attenuate large out-of-band interferers. These filters are typically mechanically-resonant devices, such as surface acoustic wave (SAW) filters, that provide a high quality factor (Q) required by many of today's communication standards (e.g., GSM). The Q-factor of a tuned circuit, such as a band-pass filter, is the ratio of its resonant frequency (or center frequency) to its 3 dB frequency bandwidth. SAW filters are generally not amenable to monolithic integration on a semiconductor substrate with the RF receiver. However, SAW filters remain conventional in RF receiver design because of the limited Q-factor of silicon-based inductors.
Although SAW filters can provide excellent attenuation of large out-of-band interferers and accurate pass-band location, they have several associated disadvantages. First, these filters have an approximate insertion loss of 1-2 dB in their pass-band. This directly adds to the noise factor and degrades sensitivity of the RF receiver. Second, these filters invariably add cost and circuit board area, especially in multi-band applications where several of these filters can be required. Third, these filters remove the flexibility of sharing LNAs in multi-band applications.
Therefore, there exists a need for an apparatus that provides adequate attenuation of large out-of-band interferers on a common semiconductor substrate, while allowing LNAs to be shared in multi-band applications.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.